Equilibrium Swelling of Biocompatible Thermo-Responsive Copolymer Gels.

Aleksey D Drozdov

doi:10.3390/gels7020040

Abstract

Biomedical applications of thermo-responsive (TR) hydrogels require these materials to be biocompatible, non-cytotoxic, and non-immunogenic. Due to serious concerns regarding potential toxicity of poly(N-isopropylacrylamide) (PNIPAm), design of alternative homo- and copolymer gels with controllable swelling properties has recently become a hot topic. This study focuses on equilibrium swelling of five potential candidates to replace PNIPAm in biomedical and biotechnological applications: poly(N-vinylcaprolactam), poly(vinyl methyl ether), poly(-dimethyl amino ethyl methacrylate), and two families of poly(2-oxazoline)s, and poly(oligo(ethylene glycol) methacrylates). To evaluate their water uptake properties and to compare them with those of substituted acrylamide gels, a unified model is developed for equilibrium swelling of TR copolymer gels with various types of swelling diagrams. Depending on the strength of hydrophobic interactions (high, intermediate, and low), the (co)polymers under consideration are split into three groups that reveal different responses at and above the volume phase transition temperature.

Highlights

To examine the ability of the model to describe equilibrium swelling of Poly(N-vinyl caprolactam) (PVCL) copolymer gels, we focus on observations on poly(N-vinylcaprolactam-co-2-methoxyethyl acrylate) P(VCL-MEA) microgels (Figure 4)
This study deals with the analysis of equilibrium swelling of thermo-responsive homo- and copolymer gels to be employed in biomedical applications
Due to the concern regarding cytotoxicity of substituted acrylamide gels, we focus on the response of five groups of biocompatible hydrogels with poly(vinylcaprolactam), poly(vinyl methyl ether), poly(dimethylaminoethyl methacrylate), poly(oxazoline)s, and poly(methoxyethoxy ethyl methacrylate) as temperature-sensitive monomers

Summary

Introduction

Thermo-responsive (TR) gels form to a special group of stimuli-sensitive hydrogels whose equilibrium water uptake is strongly affected by temperature T. The temperature at which the aggregation process starts is identified as the volume phase transition temperature Tc. As all cage-like structures formed by water molecules around hydrophobic segments are broken above VPTT, further growth of hydrophobicity of the network does not occurs, and the FH parameter χ remains equal to its ultimate value χmax. Two reasons for the disappearance of VPT can be mentioned: (i) elasticity of the polymer network that resists formation of hydrophobic clusters and hinders the aggregation process, and (ii) a strong decay in the equilibrium degree of swelling with temperature before the hydrophobicity of the network (characterized by the FH parameter χ) reaches its critical value χmax In the latter case, most water molecules are expelled from the gel before concentration of “released” hydrophobic segments becomes critical, and the driving force for aggregation disappears. PVME and PDMAEMA gels (with hydrophobic interactions of intermediate strengths) reveal transition from the swollen to the sponge-like state at the critical temperature Tc , whereas (iii) poly(2-oxazoline)s and poly(methoxyethoxy ethyl methacrylate)s demonstrate temperature-sensitivity without volume phase transition (aggregation of hydrophobic segments does not occur in these gels)

Swelling on Homopolymer Gels

Gels with Strong Hydrophobic Interactions

Gels with Weak Hydrophobic Interactions

N-Substituted Polyacrylamide Homo- and Copolymer Gels

Discussion and Conclusions